Monday, July 27, 2015

Heat Recovery Ventilators Explained

What the heck is an HRV?  Or an ERV for that matter?

If you know much about me and my writings, you are aware that I attach considerable importance to providing outside air to a living environment.  One problem with bringing in outside air is that it increases costs.  Certainly you wouldn’t think of opening two or three windows when it is 100 degrees outside – or 30 degrees?  Cooling or heating that outside air can get quite expensive.  Financially that is pretty much what is happening when you bring outside air in via the Heating, Ventilating and Air Conditioning (HVAC) system.  

The two major advantages are providing good air change, thereby diluting contaminants, and reducing or eliminating unfiltered air infiltration.  My position has pretty typically been that, like virtually any decisions in life, you have to weigh pros and cons – advantages verses disadvantages.  In this case it would be weighing health considerations against the added expense.  Since health costs are not as readily observable as the higher utility bill, many people will opt for bringing the heating and cooling costs down.  

What if there was a way to increase air change in a living environment by increasing outside air without dramatically increasing cooling and heating costs?  The answer to that scenario is a device known as a Heat Recovery Ventilator or HRV.  

An HRV operates as a heat exchanger by causing exhausted air from the house to be proximate with air coming in from outside thereby enabling cooler air to gain heat from the warmer air.  If air coming in from outdoors is cooler than indoors it is warmed by the air being exhausted from indoors.  The converse is also true.  If the air from outdoors is warmer than indoors, it is cooled by the air being exhausted.  The amount of heat recovered generally ranges between 60% and 85% depending on the type and model employed.

As heating and cooling costs have increased over the years, the emphasis has been directed toward tighter and tighter houses and buildings.  Insulation and weather stripping, more efficient doors and windows have been promoted and utilized to keep the outdoor environment outside.  Using an HRV you can add outside air to increase air change without extensive increase in utility costs.

With the known advantages of providing outside air via the HVAC system, there is one situation with the HRV that needs to be addressed.  It is an obvious one if you are familiar with one of the advantages of outside air which is to positively pressurize the house.  The HRV is generally designed to exhaust as much air as that brought in from outside.  This purportedly equalizes the pressure between inside and outside.  The problem with that is to quote some IAQ practitioner (sorry, I don’t remember who), “most houses suck.”  Bathroom and kitchen exhausts are primary guilty parties but “stack effect,” i.e. the phenomenon of warm air rising can also be a factor.  Due to air being pushed out of the house in some areas means the air has to be made up by infiltration from other locations.  This, infiltration of unfiltered air can be problematic when pulling air through under house crawl spaces or attics or any other locations including insulated walls.  Since good air change is being achieved due to the actions of the HRV, contaminants from air infiltration will be substantially mitigated by dilution, but having the house slightly positively pressurized is more ideal.  

The positive pressurization causes air to flow out (exfitration) where it would flow in due to negative pressure. To accomplish positive pressurization of the house requires having an HRV with variable speed fans that can be individually adjusted.  Adjusting the fans so that a bit more air is brought in from outside than is exhausted can achieve that positive pressure.

An additional factor that needs to be considered regarding air quality is humidity.  Certainly humidity is of more concern in some areas of the country than in others and at certain times of the year as opposed to others.  The low humidity during the winter, especially in colder areas of the country, can present some very uncomfortable issues.  Hot and high humidity environments present quite a different problem during the summer.  There is a solution.  Another version of the HRV is the ERV or Energy Recovery Ventilator.  

The ERV can transfer both heat and humidity from incoming air to outgoing air or from outgoing air to incoming air.  This action enables both heat and humidity control.  With high humidity levels during the heat of summer, both heat and humidity can be transferred from incoming air to the indoor air as it is exhausted.  With low humidity levels in the cold of winter, both heat and humidity can be transferred from stale indoor air as it is exhausted to the fresh incoming air.  So with this function air becomes more comfortable, more healthful and energy costs do not skyrocket.

To review – HRVs transfer heat between incoming outside air and outgoing indoor air.  ERVs transfer both heat and humidity between incoming outside air and outgoing indoor air.  Both HRVs and ERVs have separate fans for incoming and outgoing air that should have the ability to be separately controlled.  The fans should be set to put the house under slight positive pressure to reduce or eliminate infiltration of unfiltered air from potentially contaminated areas.  

Always use experienced professionals when deciding to install an HRV or ERV.  Proper installation, including the location of the equipment and intake and exhaust areas must be correct for the HRV or ERV to operate as intended.


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